The present invention relates to an optical film which has excellent anisotropy in the scattering of linearly polarized light and excellent thermal stability and is suitable for use in improving the perceptibility, brightness, durability, and other performances of liquid-crystal displays and the like. The present invention also relates to an optical member comprising the optical film. The invention further relates to an optical element comprising the optical film.
Conventional optical films which comprise a matrix and dispersedly contained therein regions having anisotropy in refractive index and are anisotropic with respect to the scattering of linearly polarized light include a film comprising a combination of a thermoplastic resin and a low molecular weight liquid crystal, a film comprising a combination of a low molecular weight liquid crystal and a photocrosslinkable low molecular weight liquid crystal, a film comprising a combination of a polyester and either an acrylic resin or polystyrene and a film comprising a combination of poly(vinyl alcohol) and a low molecular liquid crystal (see U.S. Pat. No. 2,123,901, WO 87/01822, EP 050617, WO 97/32224, WO 97/41484, and JP-A-9-274108). (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d.)
The optical films described above are intended to, for example, improve the perceptibility, brightness and other performances of liquid crystal displays or the like based on their polarizing/separating function and light-diffusing function both attributable to their property of anisotropically scattering linearly polarized light. However, the conventional optical films have problems that production thereof is troublesome and they are insufficient in stability required for practical use, such as heat resistance.
One object of the invention is to provide a practical optical film which has excellent anisotropy in the scattering of linearly polarized light, can be easily produced, and has excellent thermal stability.
Another object of the invention is to provide an optical member comprising the optical film.
Further object of the invention is to provide an optical element comprising the optical film.
The optical film according to the invention comprises a birefringent resin film and birefringent minute regions dispersedly contained therein, wherein the minute regions comprise a thermoplastic resin having a glass transition temperature of 50xc2x0 C. or higher and showing a nematic liquid crystal phase in a range of temperatures lower than the glass transition temperature of the resin constituting the resin film, and the difference in refractive index between the resin film and the minute regions in a direction perpendicular to the axis direction in which a linearly polarized light has a maximum transmittance, xcex94n1, is 0.03 or larger and that in the maximum-transmittance axis direction, xcex94n2, is 50% or smaller of the xcex94n1.
The optical member according to the present invention comprises two or more layers of the optical film which are superposed such that the xcex94n1 directions for any of the layers are parallel to those for the adjacent layers.
The optical element according to the present invention comprises a multilayer structure which comprises at least one of a polarizer plate and a retardation film and one or more layers of the optical film described above.
The optical film according to the present invention has the following excellent anisotropy in scattering. In the axis direction in which a linearly polarized light has a maximum transmittance (xcex94n2 direction), the linearly polarized light passes through the optical film while satisfactorily retaining its polarized state. In directions (xcex94n1 directions) perpendicular to the xcex94n2 direction, the linearly polarized light is scattered based on the difference in refractive index xcex94n1 between the resin film and the minute regions, whereby the polarized state is diminished or eliminated.
Furthermore, since the minute regions and the film dispersedly containing the same are formed from resins having excellent handleability, the optical film can be easily produced. The optical film obtained has excellent thermal stability, stably retains its optical functions over long, and has excellent suitability for practical use. According to the present invention, an optical film which undergoes no change in appearance or optical properties, e.g., scattering properties, even at temperatures as high as 80xc2x0 C. or above can be formed.
As a result, the optical film can be used to obtain a liquid-crystal display in which light loss by absorption and heat generation by light absorption can be prevented based on the polarizing properties of the optical film attributable to anisotropy in scattering, and which is excellent not only in abrightness and perceptibility but in the thermal stability of optical functions due to the satisfactory heat resistance of the optical film and has heat resistance sufficient for practical use.